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feat: add pagination support for listRoots operation #336

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feat: add pagination support for listRoots operation #336

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555c16a
feat: add pagination support for listRoots operation
tzolov Jun 24, 2025
4aeeb1f
refactor: make listRoots() return unmodifiable list and add comprehen…
tzolov Jun 24, 2025
7bc8726
test: add mock verification to McpAsyncServerExchangeTests
tzolov Jun 26, 2025
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13 changes: 12 additions & 1 deletion mcp/src/main/java/io/modelcontextprotocol/server/McpAsyncServerExchange.java
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Original file line number Diff line number Diff line change
Expand Up @@ -4,6 +4,8 @@

package io.modelcontextprotocol.server;

import java.util.ArrayList;

import com.fasterxml.jackson.core.type.TypeReference;
import io.modelcontextprotocol.spec.McpError;
import io.modelcontextprotocol.spec.McpSchema;
Expand Down Expand Up @@ -126,7 +128,16 @@ public Mono<McpSchema.ElicitResult> createElicitation(McpSchema.ElicitRequest el
* @return A Mono that emits the list of roots result.
*/
public Mono<McpSchema.ListRootsResult> listRoots() {
return this.listRoots(null);

return this.listRoots(McpSchema.FIRST_PAGE).expand(result -> {
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There are no tests for this? Can we have a test for 0 pages, 1 page, 5 pages? Network error after 3rd page?

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I've added a new Test for the aync exchange.

if (result.nextCursor() != null) {
return this.listRoots(result.nextCursor());
}
return Mono.empty();
}).reduce(new McpSchema.ListRootsResult(new ArrayList<>(), null), (allRootssResult, result) -> {
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Suggested change
}).reduce(new McpSchema.ListRootsResult(new ArrayList<>(), null), (allRootssResult, result) -> {
}).reduce(new McpSchema.ListRootsResult(new ArrayList<>(), null), (allRootsResult, result) -> {

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done

allRootssResult.roots().addAll(result.roots());
return allRootssResult;
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The backing list needs to be made unmodifiable after being accumulated.

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I made some adjustments , but not sure if this is what you've meant.

});
}

/**
Expand Down
10 changes: 9 additions & 1 deletion mcp/src/main/java/io/modelcontextprotocol/spec/McpSchema.java
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Expand Up @@ -1615,11 +1615,19 @@ public record Root( // @formatter:off
*
* @param roots An array of Root objects, each representing a root directory or file
* that the server can operate on.
* @param nextCursor An optional cursor for pagination. If present, indicates there
* are more roots available. The client can use this cursor to request the next page
* of results by sending a roots/list request with the cursor parameter set to this
*/
@JsonInclude(JsonInclude.Include.NON_ABSENT)
@JsonIgnoreProperties(ignoreUnknown = true)
public record ListRootsResult( // @formatter:off
@JsonProperty("roots") List<Root> roots) {
@JsonProperty("roots") List<Root> roots,
@JsonProperty("nextCursor") String nextCursor) {

public ListRootsResult(List<Root> roots) {
this(roots, null);
}
} // @formatter:on

}
4 changes: 2 additions & 2 deletions mcp/src/test/java/io/modelcontextprotocol/spec/McpSchemaTests.java
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Expand Up @@ -984,7 +984,7 @@ void testListRootsResult() throws Exception {

McpSchema.Root root2 = new McpSchema.Root("file:///path/to/root2", "Second Root");

McpSchema.ListRootsResult result = new McpSchema.ListRootsResult(Arrays.asList(root1, root2));
McpSchema.ListRootsResult result = new McpSchema.ListRootsResult(Arrays.asList(root1, root2), "next-cursor");

String value = mapper.writeValueAsString(result);

Expand All @@ -993,7 +993,7 @@ void testListRootsResult() throws Exception {
.isObject()
.isEqualTo(
json("""
{"roots":[{"uri":"file:///path/to/root1","name":"First Root"},{"uri":"file:///path/to/root2","name":"Second Root"}]}"""));
{"roots":[{"uri":"file:///path/to/root1","name":"First Root"},{"uri":"file:///path/to/root2","name":"Second Root"}],"nextCursor":"next-cursor"}"""));

}

Expand Down